i have a 19" 5:4 LCD in my cabinet, and you know what? it looks great. Nobody has ever complained about the way it looks, and I am quite happy with the picture on it. Not everyone needs their arcade games to be pixel perfect to the original. i have only played a small collection of real arcade games in my life...most of the games i fell in love with in mame, i have never seen before in a real arcade.
yeah, they may not look exactly like they did, but they still look great...and playing them on a full size cabinet with the proper controls makes them awesome.
if you want an LCD monitor and you are OK with the way it looks, then dont let anyone else tell you it is wrong.